Discarded event is a warning that can happen at runtime, i.e., when an application is executed.

The warning means that the event is received, but the application is not in a state that expects it. Therefore, the event is not handled.
Most of the time, this is an indication that we forget to keep an event reception active for further events. If this is the case, then find the event reception in the behavior specification and from its context menu select option Repeat.